riscv: move errata/ and kvm/ builds to arch/riscv/Kbuild

Commit 909548d6

 ("riscv: add arch/riscv/Kbuild") intended that
subdirectories in arch/riscv/ would be added in arch/riscv/Kbuild.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>